Chapter 14, End of Chapter, Review Questions, Exercise 1

The descending spinal tract involves the pathways that help in communication between the brain and the spinal cord. These pathways are divided into two major categories: lateral and ventromedial.

Lateral descending spinal pathway

It is composed of  the corticospinal and rubrospinal tracts, and involved in regulating the conscious movement of the distal musculature. 

The corticospinal tract is the longest pathway in the central nervous system (CNS) that originates from the neocortex. Some of the axons from the tract originate from the temporal lobe and constitute the motor cortex. Rest of the axons arise from the somatosensory region of the parietal lobe, and are involved in relaying somatosensory information to the brain. The axons cross the midbrain through the cerebral peduncle, and reach the pons from where they form a tract at the base of the medulla. The tract appears as a bulge known as medullary pyramid, and is also called pyramidal tract due to the appearance of a triangular shape in its cross-sectional view. The pyramidal tract crosses the pyramidal decussation at the border of medulla and spinal cord, hence the right and left motor cortex regulate their respective opposite sides of the body. The corticospinal tract ends at the dorsoventral part of the ventral horn and intermediate gray matter. 

Another component of the lateral pathway, the rubrospinal tract arises from the red nucleus of the midbrain. The axons from the red nucleus cross the pons and travel along the cerebrospinal tract to terminate at the lateral column of the spinal cord.

Ventromedial descending spinal pathway

It is composed of the medullary reticulospinal, pontine reticulospinal, vestibulospinal, and tectospinal tracts. All the pathways of the ventromedial descending tracts originate in the brainstem and end within the interneurons of the spinal cord. Both the  reticulospinal tracts originate from the reticular formation in the brainstem. The medullary reticulospinal tract descends laterally from the brainstem, and is involved in diminishing antigravity reflexes. The pontine reticulospinal tract descends medially from the brainstem, and is involved in the enhancement of the antigravity reflexes. The vestibulospinal tract arises from the vestibular nuclei of the medulla. The tectospinal tract arises from the superior colliculus of the midbrain. Both the tracts help in keeping the head balanced during movement, and also aids in the movement of head in the direction of a sensory stimulus.